This invention refers to a magnetic gripping apparatus to be used both for anchoring workpieces to machine tools and for hoisting and similar apparatus.
Magnetic gripping devices or apparatuses of flux reversal type are generally known in which a magnetic circuit is activated and de-activated by reversing the magnetization of part of the permanent magnets of the device. For example, from U.S. Pat. Nos. 4,090,162 and 4,356,467 by the same inventors it is known to provide magnetic gripping devices comprising a base plate, peripheral walls and pole units having main pole pieces defining a work-holding or gripping surface, in which the pole pieces are energized by reversible permanent magnets positioned between each main pole member and the base plate and by secondary permanent magnets disposed between the intermediate pole members and said main pole pieces, said secondary magnets being magnetized so as to present a pole of the same polarity to a corresponding pole member, and in which the reversible magnets are encircled by flux reversal coils of a first electric circuit connected to a control unit for activating and de-activating the device.
It is likewise known that in high-alloy workpieces magnetically anchored by using devices of the above-mentioned type, upon deactivation there exist a magnetic remanence which extends to hold the workpiece gripped even after the device has been de-activated, the relevant gripping force being determined by the dimensions of the workpiece as well as by the same magnetic remanence. Clearly, in certain applications, especially on grinders and hoisting apparatus the presence of a magnetic remanence in the workpiece is a disadvantage since it prevents the detachment of the workpiece and hampers the progress of normal working, or sometimes causes damage to the surfaces already machined if, in order to be removed, the workpiece has to be slid or dragged against the gripping surface. This magnetic remanence can not be removed in existing devices due to the impossibility of using activating/de-activating circuits to establish a magnetic field for totally demagnetizing the workpieces, because the same magnetic circuit would inevitably be damaged as a result.
An object of this invention is to provide a magnetic gripping device of the above-mentioned type comprising means for totally demagnetizing the gripped workpiece so that any magnetic remanence in the workpiece is entirely nullified following de-activation of the gripping device.
Another object of this invention is to provide a magnetic device, as define above, in which demagnetization of the workpiece can be achieved without interfering with or damaging the permanent magnets of the main activating circuit.
A further object of this invention is to provide a magnetic gripping device in which use is made of an appropriate auxiliary workpiece-demagnetiszng circuit, which makes use of part of main magnetic circuit of the device without damaging the permanent magnets and which at the same time does not necessitate an increase in the size or overall dimensions.